A fuel cell can convert chemical energy to electrical energy by promoting a chemical reaction between two gases.
One type of fuel cell includes a cathode flow field plate, an anode flow field plate, a membrane electrode assembly disposed between the cathode flow field plate and the anode flow field plate, and two gas diffusion layers disposed between the cathode flow field plate and the anode flow field plate. A fuel cell can also include one or more coolant flow field plates disposed adjacent the exterior of the anode flow field plate and/or the exterior of the cathode flow field plate.
Each flow field plate has an inlet region, an outlet region and open-faced channels connecting the inlet region to the outlet region and providing a way for distributing the gases to the membrane electrode assembly.
The membrane electrode assembly usually includes a solid electrolyte (e.g., a proton exchange membrane, commonly abbreviated as a PEM) between a first catalyst and a second catalyst. One gas diffusion layer is between the first catalyst and the anode flow field plate, and the other gas diffusion layer is between the second catalyst and the cathode flow field plate.
During operation of the fuel cell, one of-the gases (the anode gas) enters the anode flow field plate at the inlet region of the anode flow field plate and flows through the channels of the anode flow field plate toward the outlet region of the anode flow field plate. The other gas (the cathode gas) enters the cathode flow field plate at the inlet region of the cathode flow field plate and flows through the channels of the cathode flow field plate toward the cathode flow field plate outlet region.
As the anode gas flows through the channels of the anode flow field plate, the anode gas passes through the anode gas diffusion layer and interacts with the anode catalyst. Similarly, as the cathode gas flows through the channels of the cathode flow field plate, the cathode gas passes through the cathode gas diffusion layer and interacts with the cathode catalyst.
The anode catalyst interacts with the anbode gas to catalyze the conversion of the anode gas to reaction intermediates. The reaction:intermediates include ions and electrons. The cathode catalyst interacts with the cathode gas and the reaction intermediates to catalyze the conversion of the cathode gas to the chemical product of the fuel cell reaction.
The chemical product of the fuel cell reaction flows through a gas diffusion layer to the channels of a flow field plate (e.g., the cathode flow field plate). The chemical product then flows along the channels of the flow field plate toward the outlet region of the flow field plate.
The electrolyte provides a barrier to the flow of the electrons and gases from one side of the membrane electrode assembly to the other side of the membrane electrode assembly. However, the electrolyte allows ionic reaction intermediates to flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly.
Therefore, the ionic reaction intermediates can flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly without exiting the fuel cell. In contrast, the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly by electrically connecting an external load between the anode flow field plate and the cathode flow field plate. The external load allows the electrons to flow from the anode side of the membrane electrode assembly, through the anode flow field plate, through the load and to the cathode flow field plate.
Because electrons are formed at the anode side of the membrane electrode assembly, that means the anode gas undergoes oxidation during the fuel cell reaction. Because electrons are consumed at the cathode side of the membrane electrode assembly, that means the cathode gas undergoes reduction during the fuel cell reaction.
For example, when hydrogen and oxygen are the gases used in a fuel cell, the hydrogen flows through the anode flow field plate and undergoes oxidation. The oxygen flows through the cathode flow field plate and undergoes reduction. The specific reactions that occur in the fuel cell are represented in equations 1-3.
H2xe2x86x922H+2e31xe2x80x83xe2x80x83(1)
xc2xdO2+2H++2exe2x88x92xe2x86x92H2Oxe2x80x83xe2x80x83(2)
H2+xc2xdO2xe2x86x92H2Oxe2x80x83xe2x80x83(3)
As shown in equation 1, the hydrogen forms protons (H+) and electrons. The protons flow through the electrolyte to the cathode side of the membrane electrode assembly, and the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly through the external load. As shown in equation 2, the electrons and protons react with the oxygen to,form water. Equation 3 shows the overall fuel cell reaction.
In addition to forming chemical products, the fuel cell reaction produces heat. One or more coolant flow field plates are typically used to conduct the heat away from the fuel cell and prevent it from overheating.
Each coolant flow field plate has an inlet region, an outlet region and channels that provide fluid communication between the coolant flow field plate inlet region and the coolant flow field plate outlet region. A coolant (e.g., liquid de-ionized water) at a relatively low temperature enters the coolant flow field plate at the inlet region, flows through the channels of the coolant flow field plate toward the outlet region of the coolant flow field plate, and exits the coolant flow field plate at the outlet region of the coolant flow field plate. As the coolant flows through the channels of the coolant flow field plate, the coolant absorbs heat formed in the fuel cell. When the coolant exits the coolant flow field plate, the heat absorbed by the coolant is removed from the fuel cell.
To increase the electrical energy available, a plurality of fuel cells can be arranged in series to form a fuel cell stack. In a fuel cell stack, one side of a flow field plate functions as the anode flow field plate for one fuel cell while the opposite side of the flow field plate functions as the cathode flow field plate in another fuel cell. This arrangement may be referred to as a bipolar plate. The stack may also include monopolar plates such as, for example, an anode coolant flow field plate having one side that serves as an anode flow field plate and another side that serves as a coolant flow field plate. As an example, the open-faced coolant channels of an anode coolant flow field plate and a cathode coolant flow field plate may be mated to form collective coolant channels to cool the adjacent flow field plates forming fuel cells.
The invention relates to a fuel cell system having a sensor.
The sensor is interfaced with the fuel cell system such that the sensor can detect the concentration of an anode gas (e.g., hydrogen) flowing in the fuel cell inlet and outlet streams. The difference in the anode gas concentration in the streams can be measured and transmitted to a controller. In response to the measured difference, the controller can regulate the flow of the anode gas in the inlet stream (e.g., by changing the position of a valve disposed between the anode gas supply and the fuel cell inlet). This can allow the flow of anode gas from the gas supply to the fuel cell to be regulated according to the amount of anode gas desired to operate the fuel cell. The sensor, for example, can be formed of a membrane electrode assembly (MEA) having a solid electrolyte between two catalyst layers. The MEA can provide a sensor that is simple and inexpensive to produce.
In one aspect, the invention features a fuel cell system that includes a fuel cell, a sensor and a detector. The fuel cell has a flow field plate with an inlet and an outlet. The sensor includes a membrane electrode assembly having a first side in fluid communication with the flow field plate inlet and a second side in fluid communication with the flow field plate outlet. The detector is in electrical communication with the first and second sides of the membrane electrode assembly, and the detector is adapted to detect a difference between the hydrogen concentrations at the first and second sides of the membrane electrode assembly.
Embodiments can include one or more of the following features.
The membrane electrode assembly can be formed of a solid electrolyte disposed between two catalyst layers (e.g., two platinum-containing layers). The membrane electrode assembly can further include two gas diffusion layers, with a gas diffusion layer adjacent the exterior of each of the catalyst layers.
The fuel cell system can further include a fuel supply system (e.g., a reformer) in fluid communication with the flow field plate inlet, and a controller adapted to regulate flow of fuel from the fuel supply system to the flow field plate (e.g., in response to a signal from the sensor). The fuel cell system can also include a valve in fluid communication with the flow field plate inlet (e.g., such that the controller is interfaced with the valve to regulate flow of fuel from the fuel supply system to the inlet of the flow field plate in response to a signal from the sensor).
The fuel cell system can further include one or more additional sensors, which can be connected in series to the first sensor.
The fuel cell system can include an inlet conduit in fluid communication with the flow field plate inlet and an outlet conduit in fluid communication with the flow field plate outlet. The first side of the membrane electrode assembly can define a portion of the inlet conduit, and the second side of the membrane electrode assembly can define a portion of the outlet conduit.
In another aspect, the invention features a fuel cell system that includes a first fuel cell stack, a second fuel cell stack, a sensor and a detector. The first fuel cell stack has a first fuel cell having a first flow field plate with an inlet and an outlet, and the second fuel cell stack has a second fuel cell with a second flow field plate having an inlet and an outlet. The inlet of the first flow field plate is in fluid communication with the inlet of the second flow field plate, and the outlet of the first flow field plate is in fluid communication with the outlet of the second flow field plate. The sensor is formed of a membrane electrode assembly having a first side in fluid communication with the inlets of the first and second flow field plates, and a second side in fluid communication with the outlets of the first and second flow field plate. The detector is in electrical communication with the first and second sides of the membrane electrode assembly, and the detector is adapted to detect a difference in the hydrogen concentrations at the first and second sides of the membrane electrode assembly.
In a further aspect, the invention features a method of regulating a gas flow in a fuel cell system. The method includes contacting an inlet gas stream of the fuel cell with a first side of a membrane electrode assembly, and contacting an outlet gas stream of the fuel cell with a second side of the membrane electrode assembly. The method also includes detecting a difference between a hydrogen concentration at the first side of the membrane electrode assembly and a hydrogen concentration at the second side of the membrane electrode assembly.
The method can further include regulating flow of the inlet gas stream based on the difference in the hydrogen activities of the first and second sides of the membrane electrode assembly, contacting the inlet gas stream of the fuel cell with a first side of a second membrane electrode assembly, and/or contacting the inlet gas stream with an inlet of a flow field plate of a fuel cell. The inlet gas stream can contact the flow field plate inlet in parallel with the first side of the membrane electrode assembly.
Other advantages and features of the invention will be understood from the figures, detailed description and claims.